change error into warning so that 'make check' work with older (4.5.3) compiler

dune/common/test/shared_ptrtest.cc

@@ -153,8 +153,8 @@ int main(){
 
     // test constructor from nullptr
 #ifndef HAVE_NULLPTR
-        #error Construction of shared_ptr from a nullptr will not work as compiler
-        #error does not support the latter.
+        #warning Construction of shared_ptr from a nullptr will not work as compiler
+        #warning does not support the latter.
     shared_ptr<double> bar_null=shared_ptr<double>();
 #else
     shared_ptr<double> bar_null(nullptr);